As far as I know, the prequels were never mixed into matrix DTS. TPM was the premiere movie for Dolby EX matrix and DTS later premiered their ES variant.
I don’t think the discrete ES version was ever used theatrically, as it was primarily designed for home video. Neither system really took off and became more of a marketing gimmick for video. Then THX discovered a flaw that caused the ear to miscue sound and necessitated the reconfiguration into 7.1.

I’ve tried the prequels in the various decoders and they work fine. The OT being 6.1 is evidence of how lazy they are. The awful DVD mixes were in DD EX and then these were decoded back to 6.1 at the source before being further bastardized.