Worse performance after create empty table
It would help to see the slow query, and its EXPLAIN.

Meanwhile, try

ANALYZE TABLE invoice_requests;


There are no useful 'statistics' on an empty table.  InnoDB should have recalculated the stats before you got to this problem, but we may be missing something.

Indexes are a mess:

PRIMARY KEY (`REQUEST_ID`),   -- notes 1,2
UNIQUE INDEX `UNQ_INVOICE_ID` (`INVOICE_ID`),  -- notes 1,3
INDEX `IDX_REQ_CODE_END_TIME` (`REQUEST_ID`, `ENDTIME`),  -- note 2
INDEX `IDX_INV_NUMBER` (`INVOICE_ID`)  -- note 3


Notes:


Why have two unique indexes?  Can't you live with just one?  (Not knowing a "request" from an "invoice", I cannot advise further.)
Don't bother adding an index that starts with the PK's column(s)
A UNIQUE index is an index plus a uniqueness constraint.  So, don't bother adding another index with the same column(s).


Having a bunch of tables like 201805_invoice_requests will be a problem in the long run.  (There are many threads discussing having multiple identical table.)