You also need
MAX_TIMESTAMP_LOOKAHEAD. If you are sure that your settings are correct, it must be something else. If you are doing a sourcetype override/overwrite, you must use the ORIGINAL value, NOT the new value. You must deploy your settings to the first full instance(s) of Splunk that handle the events (usually either the HF tier if you use one, or else your Indexer tier) UNLESS you are using HEC's JSON endpoint (it gets pre-cooked) or INDEXED_EXTRACTIONS (configs go on the UF in that case), then restart all Splunk instances there. When (re)evaluating, you must send in new events (old events will stay broken), then test using
_index_earliest=-5m to be absolutely certain that you are only examining the newly indexed events.