Exports 1 second too long or short, never seen this before

TV broadcast, we need our edits to end exactly. On my iMac Pro at home an my 2013 rMBP I get frame precise exports.

But, at the station we have two other Macs (2016 iMac and MP trash can) that are adding or subtracting one second to every other export.

All source media is sent to us as unreadable mpg files, converted to H.264 with HandBreak, with very good results.

Anyone ever see this, have a solution, know what the hell is happening? Very unacceptable in a TV broadcast environment.

Check the:
1) Frame count in Handbrake on the unreadable (?) mpg file, (Main window, under RANGE - Chapters. Seconds. Frames.) before conversion with the

2) Frame count on the conversion after HB spits out the file with the

3) Frame count of the final edit FCP renders.

They all should match. If not run a quick test to see at which point frames gets added or removed.

Yes, all of that verified, the discrepancy is in the FCP exported file only. FCP timeline tells me I have 7 minutes 20 seconds exactly. FCP Browser tells me I have 7 minutes 20 seconds exactly. Exported file is 7 minutes 21 seconds.

If that's direct output from FCP it's be interesting to see if sending it to Compressor produces the same one second overage. Export an XML & see if there's a disconnect in the duration or offset parameters.