Fact: TNT is not supposed to blow up blocks underwater.

Remark: After placing water hacked blocks, such as kelp, seagrass, slabs underwater, and letting TNT explode inside them, we notice that the TNT blows these blocks up. This also affects other blocks that are within the range of the TNT. (In the video I used dirt blocks, as they have a low blast resistance.) The same thing happens if we drop a TNT inside a bubble columb. (In the video I only tested it with magma blocks, but this is most likely the case with soul sand based bubble columbs as well.) This behavior is most likely a bug.

Video:
Available on YouTube.

Note:
This bug is (most likely) caused by MC-125298.