Still a problem in April 2024. Recently upgraded from an iphone 4 plus to a 15 plus, the Dexcom complication HAD been dead center in an easily readable font, but afterwards showed 2 lines of dashes. Trying to pick a new, or edit the existing, watch face got a message that watchos needed to be updated. After the update, found that the Dexcom complication was no longer available in the center of ANY watch face, and the few watch faces that did contain the Dexcom complication now showed it in a tiny, virtually unreadable font. Plus, it regularly changed to a line of dashes and no longer automatically updated the CGM numbers.
This old programmer recognizes this as either poor design decisions, lack of end-user involvement in testing or just plain inadequate testing.
I agree with another post that said the 'motion face' is marginally better but then you lose more complication space and get stuck with butterflies or jellyfish.
p.s. I did reach out to Dexcom and opened a ticket with them. I'll update this post if they reply.