Small Transport
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 1.5
Communication Breakdown
ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued New Clearance
Procedure
I was working the R1 position. Aircraft X was inbound from the south requested RNAV [Runway] XX approach to get below the deck and requested to proceed via own navigation to ZZZZZ. I cleared Aircraft X to maintain and cross ZZZZZ at or above 5;700 ft.; RNAV [Runway] XX ZZZ Approach. Aircraft Y was inbound from the east; and instructed to hold east of ZZZ at 6;000 ft. for sequence behind Aircraft X. Aircraft X was radar service terminated and switched to ZZZ Tower. ZZZ Tower had not been advised of Aircraft Y in holding. At approximately 5.5 mile; ZZZ Tower called and advised Aircraft X was requesting a 360. I approved Aircraft X request. When I observed Aircraft X continuing to travel eastbound; I called ZZZ Tower to verify Aircraft X had canceled IFR. ZZZ Tower advised they had not canceled IFR. When I observed Aircraft X began to climb higher than 4;600 ft. eventually up to 5;200 ft.; I issued Aircraft Y to continue in holding except to climb to 7;000 ft. CA went into alarm; I issued a traffic alert to Aircraft Y and advised a right turn heading 360 and to climb and maintain 7;000 ft. I called ZZZ Tower and asked them to switch Aircraft X back to me. Aircraft X was still on Tower's frequency and they rejoined the RNAV [Runway] XX at 10.5 miles and continued inbound.On initial call from ZZZ Tower; I should have verified that Aircraft X had canceled IFR with Tower. I could have instructed Aircraft Y to hold at 7;000 ft. from the initial call.
Controller reported previous IFR released to Tower made unexpected climb towards holding traffic resulted in an airborne conflict.