When breeding axolotls with tropical fish or a bucket of tropical fish, there is a roughly 10-second pause where the two axolotls will simply hang motionless in the water with all their limbs in the default position. I guess this is the axolotl equivalent of a T-pose.
During this time no particles appear and if the axolotls are bumped by any other mob or the player, they will fail to breed and revert to their normal behavior. The rest of the game seemingly continues to operate as normal.
The problem is illustrated in the attached video. I don't have a specific explanation, but it appears similar to the issue in early 1.15 snapshots where bee AI would stop and they would float motionless until bumped.
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Can confirm for 20w51a

I also want to note that this is not related to the presence or absence of heart particles depending on graphics settings, and it also does not seem to be a performance-related issue. I have not had a chance to test other cases, but I observed this bug with axolotls spawned from a bucket-of-axolotls from the creative inventory. I will test later with spawn eggs and /summon to see if that makes any difference.
Can confirm in 21w03a.
Can confirm in 21w05a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 21w08b.
Can confirm in 21w11a.
Can confirm in 21w13a.
Can confirm in 21w14a.

21w16a
Mobs make particles, and reproduce near instantly. Cannot reproduce.