|timestamp returning different values

This has been previously working, and only just changed in the last week or so, but I have also had to change the machine my HA is running on.
So why do I get a different start time with the |timestamp in one calendar, but the other calendar works fine?

start: {{states.calendar.michael.attributes.start_time}}
nice: {{as_timestamp(states.calendar.michael.attributes.start_time) |timestamp_custom('%-I:%M %p',true) }}

start: {{states.calendar.michael_appointments.attributes.start_time}}
nice: {{as_timestamp(states.calendar.michael_appointments.attributes.start_time) |timestamp_custom('%-I:%M %p',true) }}

start: 2023-03-10 10:00:00
nice: 9:00 AM

start: 2023-05-31 10:40:00
nice: 10:40 AM

Edit: as per info found in other threads,
{{ now().tzinfo }}
shows the correct timezone of Australia/Brisbane
{{as_timestamp(now())|timestamp_custom('%-I:%M %p') }}
also shows the correct time.