Didn't mean to make this topic about rappy egg! (Though, if it is: "16 runs at 500% RDR and no drop now.")
Somewhere along the way they decided to make rappy 13* drop only. JP included a couple 10* versions as bonuses for separate purchases in 2016 (an Episode 4 physical edition and volume 6 of the DVD series for the animation), so they probably wanted to keep it rare during Episode 4 so those purchases (particularly the former) were more appealing. We got most of those bonuses mixed into two premium packs but neither kept the egg so we're stuck with rare 13* drops.
JP have since had limited-time quests and even collection folders (including a 14* light element in easter 2020!) so we'll probably get something similar in the medium-term future. It'd just be nice to not have to wait that long, and if Deus Esca departs the schedule in a couple days that'll pretty much ensure that wait.
Edit: I need to follow up something I said a few posts ago:
the last I saw [Rappy Egg from Crimson Fellwyrm] dropped substantially fewer relative to 10* drops than Deus Esca.
I've reviewed in the current window: We've had 321 Rappy Egg and 173,721 of the most common 10* item (Deo Laserswift). That's 541 of the most common 10* per Rappy Egg.
Deus Esca has 155 Rappy Egg and 105,584 of the most common 10* (Deo Adelaide). So that's 681 of the most common 10* per Rappy Egg. The rematch (53 and 32,924 so 621 per) broadly matches an old JP interview saying the Rappy Egg drop rate was the same between the first fight and the rematch.
So on paper Crimson Fellwyrm might be better than Deus Esca. The comparison isn't perfect (among other things, I'm pretty sure Deus Esca drops way more 10* weapons. I often come away with 25-30 cubes from the first part! It also doesn't drop as many 12* and higher items) but at the very least we can see Crimson Fellwyrm isn't suffering for Rappy Egg drops.
Additional, while there are other 13* eggs as I pointed out, Rappy is the most common one (321 Rappy, 152 Melon, 148 Marron, 146 Jinga, 145 Viola, 135 Redran 128 Synchro, 124 Popple and 119 Aero). So while a ? Special Egg only has about a 22.5% chance to be Rappy, the rest seem to be additional rather than interference.
Still, for anyone who can do both runs on Deus Esca and only the first of Crimson Fellwyrm, the former is clearly preferable.