"Regions I've visited" diseappear when adding new ones

[CouchersBot]

Subject: "Regions I've visited" diseappear when adding new ones
Description:
Ticking new "Regions I've visited" seems to sometimes randomly remove previously ticked ones. Might be when more than ~10 are ticked?

Results:
Previously selected countries get unselected when selecting new ones, (but not consistently, they don't and then 5 disappear at once.)
They shouldn't get unselected.

Backend version: develop-06099492
Frontend version: develop-06099492
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User: olip (46779)