I've opened a ticket on this but if anyone else is seeing it, more reports might help in tracking down the root cause.
I decided to post here as well in case anyone else is running into this for their area or might have an idea if it is something I'd done wrong. The missing trails do appear to be tagged in the same manner as those that were added.
Garmin Edge Trailforks map update 2021.01 did not copy all of the maps added last year. (I see a small portion)
Seen at lat/lon 38.986825 -86.270510
I had updated 18 maps to exist for Trailforks within this Hoosier National Forest (added Fall 2020).
I did mark them all as Mountain Bike trails. A few existed in TF as hiking trails and I updated them to MTB trails (because they are in fact MTB allowed trails).
They all exist in Trailforks. (top picture)
Only a few of them were copied over to the Garmin Trailforks basemaps. (Lower left picture)
If I use a paid trailforks PRO account to copy their basemaps I can overlay it on the garmin (lower right picture).
This shows (yellow trails) that are missing from the built in Garmin basemaps.
Screenshots: