The bug
Water and lava can flow trough the worldborder. As MC-54366 was fixed I believe this is a bug as well.
Linked issues
relates to
Attachments
Comments

Not really, I didn't place the water outside, it flowed there
Confirmed.

Confirmed for 1.9

Still in 16w15b
Can confirm for MC 1.12.1.
Confirmed in 1.16-pre5.

Affects 1.16 Release Candidate, 1.16, 1.16.1 and 20w27a
Can confirm in 20w48a.
Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05a.
Can confirm in 21w06a.
Can confirm in 21w15a.
Can confirm in 1.17.
Can confirm in 1.17.1.

Can confirm in 21w37a.

Can confirm in 21w38a.
Can confirm in 1.18.
Can confirm in 1.18.1.
Can confirm in 1.18.2 and 22w18a.
Can confirm in 1.19.2 and 22w42a.
Dupe of MC-54119