Sounds like you are employed at the lower grade, not on probation. Lots of people at my place get done over in a similar fashion to this with extended periods in “interim” higher graded roles, that drag on and on.
Do you have a good relationship with your line manager?
Do have any good relationships with other senior people at your place, not in your immediate line, with whom you could have a confidential “mentoring” chat?
Some good advice above about putting your case.
I would suggest that, if you can, you talk it over in confidence with someone who knows the company well. Preferably who is senior to you but with whom you have a good relationship and who you can trust to have a confidential chat. Worth it to understand the lie of the land, there could be things going on behind the seens, pressure to make cuts etc, that you don’t want to fall foul of.