SRT exports incorrect timecode

Within the last couple of weeks, my SRT timecodes have become off.  This had not been a problem but then suddenly happened whereby the file is :04 seconds behind where it should appear. 

For example, in the timeline, a particular reading appears from 1:00:00:00 to 1:00:04:00.

But in the SRT file, the timecode will show as 1:00:04:12 to 1:00:08:12 (for example).

I have tried a few things but without success, so interested to hear others ideas.

Strangely, exporting as a .txt file or .csv file is accurate but unfortunately I can't import those into YouTube.