it’s big jump. try to first upgrade to 8.4p1 then make a test. if all looks good then apply 8.5. i haven’t heard such issue yet. only one user reported After 8.5 upgrade checkSession problem - IdentityIQ (IIQ) / IIQ Discussion and Questions - SailPoint Developer Community
let me try at my end this weekend. also, sailpointclassicbundle in your case is OOB or custom ?