@Bunnyflop7 You shouldn't be worse off, but as others have said, working a Monday as a part timer can impact your holiday entitlement more than others as you are automatically receiving potentially more bank holidays than your pro rata entitlement. There are two ways to communicate this to colleagues; either a deduction made from your annual leave entitlement which you don't 'see' as such, so remaining holidays are yours to book (no need to book bank hols), or you are given the full entitlement to then book the bank holidays from. Both methods works out the same but just depends how business prefer.
I've worked using both methods, the first whereby you would pro rata your annual holiday entitlement and making any additions/ deductions from this based on your working pattern vs where the bank holidays fall each year (as Christmas, boxing and New year's day rotate and can fall on varying days of the year). In some instances those who work mid week for example don't recieve many bank hols so have additional days given to them to ensure they have the right amount.
We would then give them the final amount of days to take (didn't need to book bank holidays). This could be confusing as two colleagues who work two days could recieve different entitlements based on what days they work. I ALWAYS had the query every year why 'hers was different to mine'.
In my current organisation we work out the entitlement of holidays and bank holiday entitlement as full and give the colleagues their days to take on the basis they then need to book their bank holidays from this and this is where the differences are seen at the point of booking, but in difference to the above example two colleagues working 2 days per week would recieve the same entitlement BUT the person working Mondays would need to book more bank holidays from their entitlement because this is their working day and the benefit of receiving a full days pay on bank holiday Mondays. Same entitlement but unfortunately less choice because the bank holidays are on set days, notably Mondays.
I'd just ask your HR Team to break down down calculation so you can see this and I'm sure it would make sense. Happy to help or review this for you from another HR perspective if you are still unsure 👍