Is your feature request related to a problem? Please describe.
Water and lava flowing while pasting and due to that blocks getting skipped

Describe the solution you'd like
Handling Water and lava in a second pass so the solid blocks are already set and no outflow happens inside the pasted area

I this indeed a server problem. Thanks!

Run /tick freeze before paste schematic and run /tick unfreeze after pasting.

I'm assuming this is on a server? In single player the pasting should happen one chunk at a time, and also without any block updates, so they shouldn't flow even if they are floating source blocks.

I guess for the "fallback option" of using /setblock commands, a multi-phase pasting procedure might be needed, at least if the block replace behavior is set to None. But also if you don't specifically need that replace behavior, you could use the "With non-air" replace mode (in Generic configs, or cycle with Ctrl + M) to allow the surrounding blocks to be placed even after the fluids start flowing.

But I'll look into adding this at some point. The primary intended multiplayer pasting will be via the Servux mod at some point, which will allow proper pasting instead of resorting to the problematic and command-spammy vanilla /setblock commands. But in cases where it's not possible to use Servux on the server, the command-based pasting could definitely use a few improvements still.