[THIS IS A REPOST OF THIS POST, YET I BELIEVE IT IS PRETTY UNINTENDED.]
[SPECS ARE IN THE PRIVATE MESSAGE]
When adding a WeldConstraint to a Part like this:

It doesn’t apply in the playtest.
But once I set the Part0 and Part1 to the same things in the playtest and then set it back as the original, the WeldConstraints apply.
Otherwise they would just freeze “disconnecting” the WeldConstraint allows the part to move freely as it is unanchored.
Expected behavior
The WeldConstraint should “weld” (allow Part0 to move in sync with Part1) properly, even when setting the WeldConstraint before the playtest.
A private message is associated with this bug report