and if the pullups are on the MCU, they probably won't help much more than enabling the uart? it's still going to glitch on reset.

My next suggestion to debug would be:

* if you have a logic analyzer, listen to the line
* write the serial data to a bin file, look at it with a hex editor

Then you'd want to see if they disagree, or if you only have the bin file, see if you can decode the defmt messages manually