2014 Octavia 3 - 1.6l diesel greenline

Pls help with an odd thing earlier today - I'm on the motorway and when I was in traffic it gets hard to select gears (harder for 1-4, easier for 5-6). By the time I've left the motorway and pulled over I can no longer select 1st and it's really hard to select the others. Once I turn off the engine all gears are somewhat ok again. To me this suggests the clutch was not fully disengaging? The brake fluid reservoir was full to max.

I left the car for half an hour or so and when I came back, everything was back to normal. All gears are working smoothly and just fine and was fine like this for the rest of the day. The brake fluid reservoir looks to be at the same full level.

This is the second time this has happened in about six months. It's almost like the hydraulic system depressurised after sat on the motorway for a while with no gear changes. But how come now it seems to be ok? What's likely to be failing soon?

Likely to be a failing master or slave cylinder.