tz(timezone, true).utc() is not always changing the time to UTC

[hbj]

Describe the bug
Calling utc() without passing true as parameter should change the current time to UTC. However this doesn't always work on a dayjs object returned by tz() where the second parameter is set to true (to keep current time).

Example (Europe/Paris is GMT+1 / GMT+2 in DST):

dayjs('2021-02-01T15:00').tz('Europe/Paris', true).utc().format()
dayjs('2021-09-01T15:00').tz('Europe/Paris', true).utc().format()

returns

'2021-02-01T15:00:00Z'  => NON DST: INCORRECT
'2021-09-01T13:00:00Z'  => DST: CORRECT

Expected behavior
For non DST it should return

'2021-02-01T14:00:00Z'

Information

• Day.js Version [1.10.7]
• OS: [MacOS 11.5.2]
• Browser [Chrome 93]
• Time zone: [GMT+02:00 DST (Europe/Paris)]