My baby is about to turn 5 months next week. I place her on her back to sleep however recently starting to roll on her side to sleep and even more so lately, started to sleep on her stomach where she’s starts to nuzzle the mattress and then face s down to sleep. She’s in an arms free sleep sack and sleeping on a firm, flat mattress with nothing else in the cot. When I see her I turn her so she’s on her back but she immediately turns to the side. She can’t yet roll either way, we’ve started to increase tummy time but I’m feel scared to sleep at night in case she faces down and can’t get herself out that position. Help!

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
You have modified her sleep environment to meet this new development stage - arms free, cot (not bassinet) with firm flat mattress & nothing soft or loose in her cot.
Continue to always place your baby on back for sleep.

How does she go with tummy time when awake?
Increasing tummy time when awake (as you are doing) is important.
Is she rolling from tummy to back during tummy time, holding her head well & reaching for toys?

Some babies do face plant at times when sleeping - the firmness of the mattress & nothing soft in the cot is important when this happens.
Have you tried gently turning her face to the side?