I am running PAL games on my master system with a 50Hz switch and it works…
I’m not sure what you mean; PAL games from the Dreamcast aren’t working, but PAL games from your Master System are?
…but of course that isn’t a 625i signal.
The OSSC displays total line counts, not visible line counts; so 625i is the same as 576i–576 visible lines, 625 total. Likewise, 525i is the same as 480i–480 visible lines, 525 total.