Split rules are always applied before other rules, regardless of configured priority. For example, if you have category rules or tag rules with priority 20 and one split rule with priority 1, the split rule gets applied first.

If you change the order, and have the split rule with priority 20 and all other rules with priority 1, the split rule still gets applied first.

This means that if you have a category rule to first categorize the original transaction, and then split, it doesn't work as expected. The original transaction gets split first, and the category rule applies to the original transaction after the split already happened.

In the case where multiple split rules exist, the one with the lower number is prioritized (ie. priority 1).