I'm trying to use pmapToTranscripts to map ORFs within transcripts. I'm expecting it to go 1-to-1 through x and transcripts as described, but whenever there are multiple isoforms it seems to select the first one. Example:

dummy_x <- GRanges(seqnames = "chr1",
                   ranges = IRanges(start = c(20, 10, 5),
                                    end = c(300, 250, 400)),
                   strand = "+")
dummy_transcripts <- GRangesList(GRanges("chr1", IRanges(start = c(5, 100), end = c(95, 500)), strand = "+"),
                                 GRanges("chr1", IRanges(start = c(10, 100), end = c(90, 500)), strand = "+"),
                                 GRanges("chr1", IRanges(start = c(1, 100), end = c(95, 500)), strand = "+"))

pmapToTranscripts(dummy_x, dummy_transcripts)

GRanges object with 3 ranges and 0 metadata columns:
      seqnames    ranges strand
         <Rle> <IRanges>  <Rle>
  [1]        1    16-292      +
  [2]        1     6-242      +
  [3]        1     1-392      +
  -------
  seqinfo: 3 sequences from an unspecified genome

Is this the expected behavior? How can I get it to do what I want?

Session info:

R version 4.0.2 (2020-06-22)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 17134)

Matrix products: default

locale:
[1] LC_COLLATE=English_United States.1252  LC_CTYPE=English_United States.1252    LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C                           LC_TIME=English_United States.1252    

attached base packages:
[1] stats4    parallel  stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
 [1] IsoformSwitchAnalyzeR_1.11.3 ggplot2_3.3.2                DEXSeq_1.34.1                RColorBrewer_1.1-2           DESeq2_1.28.1               
 [6] SummarizedExperiment_1.18.2  DelayedArray_0.14.1          matrixStats_0.57.0           BiocParallel_1.22.0          limma_3.44.3                
[11] GenomicFeatures_1.40.1       AnnotationDbi_1.50.3         Biobase_2.48.0               GenomicRanges_1.40.0         GenomeInfoDb_1.24.2         
[16] IRanges_2.22.2               S4Vectors_0.26.1             BiocGenerics_0.34.0         

loaded via a namespace (and not attached):
 [1] bitops_1.0-6             bit64_4.0.5              progress_1.2.2           httr_1.4.2               tools_4.0.2              R6_2.4.1                
 [7] DBI_1.1.0                colorspace_1.4-1         withr_2.3.0              tidyselect_1.1.0         gridExtra_2.3            prettyunits_1.1.1       
[13] bit_4.0.4                curl_4.3                 compiler_4.0.2           VennDiagram_1.6.20       formatR_1.7              rtracklayer_1.48.0      
[19] scales_1.1.1             readr_1.3.1              genefilter_1.70.0        askpass_1.1              rappdirs_0.3.1           stringr_1.4.0           
[25] digest_0.6.25            Rsamtools_2.4.0          XVector_0.28.0           pkgconfig_2.0.3          dbplyr_1.4.4             BSgenome_1.56.0         
[31] rlang_0.4.7              rstudioapi_0.11          RSQLite_2.2.1            generics_0.0.2           hwriter_1.3.2            dplyr_1.0.2             
[37] RCurl_1.98-1.2           magrittr_1.5             GenomeInfoDbData_1.2.3   futile.logger_1.4.3      Matrix_1.2-18            Rcpp_1.0.5              
[43] munsell_0.5.0            lifecycle_0.2.0          edgeR_3.30.3             stringi_1.5.3            zlibbioc_1.34.0          plyr_1.8.6              
[49] BiocFileCache_1.12.1     grid_4.0.2               blob_1.2.1               crayon_1.3.4             lattice_0.20-41          DRIMSeq_1.16.1          
[55] Biostrings_2.56.0        splines_4.0.2            annotate_1.66.0          hms_0.5.3                locfit_1.5-9.4           pillar_1.4.6            
[61] geneplotter_1.66.0       reshape2_1.4.4           biomaRt_2.44.1           futile.options_1.0.1     XML_3.99-0.5             glue_1.4.2              
[67] packrat_0.5.0            lambda.r_1.2.4           vctrs_0.3.4              gtable_0.3.0             openssl_1.4.3            purrr_0.3.4             
[73] assertthat_0.2.1         xtable_1.8-4             survival_3.2-3           tibble_3.0.3             GenomicAlignments_1.24.0 memoise_1.1.0           
[79] tximport_1.16.1          statmod_1.4.34           ellipsis_0.3.1