This is bizarre and probably a Catalina bug as noted above...
Been waiting a while to buy the 16”, but last week the display on my 2010mbp died...So I was forced to buy a 2019 15"...Got everything migrated without issue...2010 was on High Sierra, 15" was on Mojave....Migrated then updated 15" to Catalina....Used the 15" for a week...Returned it today and got the 16"
So tonight I migrate into the 16" from Time Machine, everything comes fine except photos...Something happened in Catalina on the 15" that didn't bring the photo library into Time Machine....I've managed to scrounge the Time Machine disk and find an older library. Imported all the masters from there...Dont have 2019 photos, but it looks like Ive recovered the rest....I should be able to recover late 2018 and 2019 from iPhones....What a mess....