Increment by on our oracle sequence is 20, and historically allocationSize has been set to 50 and we've never had an issue.

Recently, I tried to align this with increment by and set it to 20.

In production, there are several instances of the same program running that will all share the same sequence. _SEQ

So, can the above result in constraint violations?

If you're using the pooled or pooled-lo generator, then there shall be no problem because the sequence value is included in the upper or lower boundaries of the generated sequence values.

If you're using the hilo generator, then you could risk having collisions.

Best thing, just take a copy of the production database and test it on your local machine to see if you get those identifier conflicts.

Using Hilo yes.

Sorry I was mistaken earlier. The increment by is only 1.

And I observe that I was getting negative values for the ID column in some instances.

So, question.

The risk of collisions exist when allocationSize (20) = "increment by" (1) of Oracle Sequence?

There was no risk of collision when allocationSize (50) > "increment by" (1)

The batch insert size I have is actually 50

Unfortunately it's not possible to copy production and I wasn't able to reproduce
the behavior outside of production.

So it seems like by reducing the allocationSize from 50 to 20 I invited collisions with values that were already used on the day. I didn't see these collisions in UAT because I don't
have enough data there for a collision to occur.

If I were you, I would not change the allocationSize unless you fell that you'll run out of identifiers.