Underground Biter attacks still attack regardless of hitting a biter evolution threshold for it to be normally toggled or turning the underground attack map setting off and minimizing the probability which cannot be set to 0. Even on freshly generated worlds with these settings pre-toggled. When playing with expensive technologies, increased pollution, and an island as a starting area, with the grace period only being able to set to a max of 2 hours, not being able to change the setting to not be a factor altogether is devastating.

Checked, the setting "AI: Underground attacks". It works. When setting is unchecked, current underground attacks are "frozen", new ones are not created.
If this is not the case for you, then I need a save with the setting disabled and an active ground attack. Ideally put a marker on the map so I know where to look.

Regarding the duration of the initial peaceful period - the new release will increase its max duration

https://files.catbox.moe/6otkxu.zip

They attack all throughout the night period due to having nocturnal biters on.
Edit: Does the underground attacks also involve them actively bypassing walls as well?

Not really. Biters avoid turrets and mass death sites.
Underground attacks follow the paths of normal attacks, but have a greater radius of finding weak spots. Therefore, they are able to detect the passage through the water between the defence.

If the length of the water is more than two chunks, then they will not find a path

I don't see active underground attacks in your save.
Also, there can be no underground attacks before 40% of evolution, and you have 13%

I noticed unusual biters - "small sea creatures". Perhaps it was them that you considered an underground attack.
They are invisible on the screen - but I managed to select them with the mouse on pause in the editor mode.

I must also report that the mod "marine life" and Rampant (any) are very poorly compatible. There are no outright errors, but what happens on the map can be quite strange.

After several hours of testing, I discovered the strange behavior stopped when Combat Mechanics Overhaul was disabled. vice-versa the behavior isnt present when Rampant is disabled and Combat Mechanics Overhaul is enabled. It seems that is where the incompatibility is, the Marine Life biters only acted unusual just like any other biter only when both were enabled. This issue has been persisting since one of the Rampant updates in December across multiple saves, and after taking a break and coming back now, it seems that has been the only outlier that doesn't produce that strange behavior.
Thank you for facilitating in exploring the issue and the active engagement and helpfulness.