Tasks are deleted by sync (even with "a wins" update rule for task calendar)

I'm syncing two remote CalDAV storages, on Nextcloud and a service called Zoho. My CalDAV tasks are being deleted in Nextcloud when vdirsyncer runs. This is despite even setting nextcloud as "a wins" in the config file for conflicts. The calendar events seem to sync fine.

• Your vdirsyncer version

vdirsyncer, version 0.16.7 on Ubuntu 20.04 installed using apt

• If applicable, which server software (and which version) you're using

Nextcloud 26.0.2 with calendar App version 4.4.2
Zoho Mail Calendar

• Your Python version

$ python --version
Python 2.7.18
$ python3 --version
Python 3.8.10

• Your operating system

Ubuntu 20.04

• Your config file

[general]
status_path = "~/.vdirsyncer/status/"

####            <redacted_username_1>            ####
[pair <redacted_username_1>_nextcloud_zoho_cals]
a = "<redacted_username_1>_nextcloud_cal"
b = "<redacted_username_1>_zoho_cal"
collections = [["<redacted_username_1>_nczcals", "pes", "events"]]
conflict_resolution = "a wins"

[storage <redacted_username_1>_nextcloud_cal]
type = "caldav"
url = "https://<redacted>.com/remote.php/dav/calendars/<redacted_username_1>/pes/"
username = "<redacted_username_1>"
password = "<redacted>"
start_date = "datetime.now() - timedelta(days=7)"
end_date = "datetime.now() + timedelta(days=720)"

[storage <redacted_username_1>_zoho_cal]
type = "caldav"
url = "http://calendar.zoho.eu/caldav/f55a576f7332690a4afff2682af5a52b3f8857ebba5e8fa86a4d05779d7934ddbfbf7076e13193991c60ae49669ff0a6/events/"
username = "<redacted_username_1>"
password = "<redacted>"
start_date = "datetime.now() - timedelta(days=7)"
end_date = "datetime.now() + timedelta(days=720)"

####            <redacted_username_2>           ####
[pair <redacted_username_2>_nextcloud_zoho_cals]
a = "<redacted_username_2>_nextcloud_cal"
b = "<redacted_username_2>_zoho_cal"
collections = [["<redacted_username_2>_nczcals", "personal", "events"]]
conflict_resolution = "a wins"


[storage <redacted_username_2>_nextcloud_cal]
type = "caldav"
url = "https://<redacted>.com/remote.php/dav/calendars/<redacted_username_2>/personal/"
username = "<redacted_username_2>"
password = "<redacted>"
start_date = "datetime.now() - timedelta(days=7)"
end_date = "datetime.now() + timedelta(days=720)"


[storage <redacted_username_2>_zoho_cal]
type = "caldav"
url = "http://calendar.zoho.eu/caldav/81c76a403e9dd9d2b54d890c95a01f1a05d7493b0e01d6702273c660efa51fdda6df450832b2ea83588d9e9db5d49fd0/events/"
username = "<redacted_username_2>"
password = "<redacted>"
start_date = "datetime.now() - timedelta(days=7)"
end_date = "datetime.now() + timedelta(days=720)"

• Use vdirsyncer -vdebug for debug output. The output is sensitive, but
  please attach at least the last few lines before the error (if applicable),
  censored as necessary. This is almost always the most useful information.

Not applicable, there's no error

Start by reproducing the sync with the supplied Nextcloud and Zoho configuration, then capture the vdirsyncer -vdebug output; none is included here. Compare task synchronization with the working event synchronization and inspect how the "a wins" rule handles deletions. Done means tasks remain in Nextcloud and the behavior is covered by a reproducible test or diagnostic case.