[R-pkg-devel] cannot xtfrm data frames WARNING with R-devel

Dear R developers,
I am facing an issue while submitting update to my package PGRdup (https://github.com/aravind-j/PGRdup).
With R-devel Debian (2021-02-05 r7994) on submission a WARNING was reported as follows. .
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.Quitting from lines 536-542 (Introduction.Rmd) 
Error: processing vignette ?Introduction.Rmd? failed with diagnostics:
cannot xtfrm data frames
--- failed re-building ?Introduction.Rmd?
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.
.
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I am unable to replicate the same locally with the same version (2021-02-05 r79941) in Ubuntu 18.
Is this a false positive ? How to resolve this issue for successful submission.


Best Regards
J. Aravind