I have not heard of similar. Personally, I never use MP3 (except for tiny SFX files), and always convert to PCM/WAV 48KHz 16-bit in Audition. That WAV is what I Import and have never had an issue. MP3's (and AIFF's) are notorious for causing issues. That is why they never get Imported.
Now, with your conversion, did you do the Save_As PCM/WAV, or back to MP3?
I saved them back both as 16 and 24 bit PCM/WAVs. I finally had to ask the client to resubmit them as 48kHz WAV files which, naturally, solved the issue.
I thought that Premiere made an attempt to resample audio files which differ from the targeted frequency of the project.
As I said in my original post, in my 12 or so years editing with Premiere, I have not been aware of encountering this before!
All very strange.
Normally, PrPro does do a good job or resampling to 48KHz 16-bt, and then conforming to 32-bit, floating point for editing. Sometimes, however, the "magic doesn't work." It is often something about the particular files, that seem to be at fault.
Glad that you got it sorted to complete the job, but sorry that I cannot pin-point why it did not function, per usual.