Small Aircraft
Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Flight Crew Last 90 Days 12; Flight Crew Total 6300; Flight Crew Type 400
Training / Qualification; Human-Machine Interface
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Environment - Non Weather Related; Human Factors
While descending on the ZZZ1 arrival we were cleared to FL290.  While passing FL300 ATC issued a pilot discretion descent clearance to 17;000 feet.  My PNF waited to set 17;000 feet until the autopilot went into ALT CAP and began to capture FL290.  Upon setting 17;000 feet I noticed the autopilot no longer holding FL290 and disconnected the autopilot and manually held altitude and then selected manual ALT to hold FL290.  At that point the autopilot was forgotten to be reengaged; and the aircraft slowly climbed to almost 29;800 feet before the PF noticed it and manually initiated descent to 17;000 feet.  ATC was notified of the correction and that we were beginning our discretion descent from current altitude to 17;000 feet.  ATC did not say anything about the brief climb and only verified we were beginning our descent to cross ZZZZZ [Intersection] at 17;000 feet.Human factors of disconnecting the automation while correcting for the unanticipated non capture; manually setting ALT hold without resetting the ALT selector caused no bug to appear on the altitude tape and the action of selecting ALT hold caused the PF to be the live autopilot; holding altitude; although only the FD was since the autopilot had not been reengaged.  Usually we would correct an autopilot behavior such as this while still engaged by resetting the ALT selector and using VS or Pitch mode; but that level of automation had been removed.  Problem was identified and quickly discussed and understood by the flight crew to ensure better communication and pilot automation interface in the future. A likely contributing factor is lack of flying hours in the recent 6 months for both crewmembers due to the COVID-19 pandemic.
Corporate Captain reported nonstandard procedures were used causing an attitude deviation. Captain stated lack of flying due to the COVID-19 pandemic was a contributing factor.