Can not send emails with delimiter in local part (Postfix + LDAP)

I just moved my mail server. With the move, an update from Postfix 3.1 to 3.4 happened (Ubuntu 16.04 to 20.04).

On the Postfix 3.1 install, I was able to send and receive emails with the delimiter + in the local part. The same config with Postfix 3.4 lets me receive emails with + in the local part but when I try sending an email with + (say, e.g. me+test@mydomain.tld) the server returns

5.7.1 <holger+testmail@(redacted)>: Sender address rejected: not owned by user

In my main.cf, the virtual maps are mapped as follows:

virtual_alias_maps = 
  hash:/etc/aliases,
  proxy:ldap:/etc/postfix/ldap-forwarding.cf, 
  proxy:ldap:/etc/postfix/ldap-aliases.cf
virtual_mailbox_domains = proxy:ldap:/etc/postfix/ldap-domains.cf
virtual_mailbox_maps = ldap:/etc/postfix/ldap-virtual-maps.cf

The sender restrictions are:

smtpd_sender_login_maps = unionmap:{proxy:ldap:/etc/postfix/ldap-aliases.cf,pcre:/etc/postfix/sender_login_override.pcre}
smtpd_sender_restrictions = permit_mynetworks, 
  reject_authenticated_sender_login_mismatch, 
  permit_sasl_authenticated, 
  reject_non_fqdn_sender, 
  reject_unknown_sender_domain, 
  reject_unlisted_sender,
  reject_unauth_pipelining, 
  reject_unknown_reverse_client_hostname, 
  reject_unknown_client_hostname, 

And, of course, the delimiter is set:

recipient_delimiter = +

ldap-forwarding.cf:

version = 3
server_host = localhost
server_port = 389

bind_dn = (redacted)
bind_pw = (redacted)
search_base = ou=Users,dc=(redacted)
query_filter = (&(|(uid=%u)(mail=%s)(mailAlternateAddress=%s))(objectClass=qmailUser)(accountStatus=forward))
result_attribute = mailForwardingAddress

ldap_aliases.cf:

version = 3
server_host = localhost
server_port = 389

bind_dn = (redacted)
bind_pw = (redacted)
search_base = ou=Users,dc=(redacted)
query_filter = (&(|(uid=%u)(mailAlternateAddress=%s)(mail=%s))(objectClass=qmailUser)(|(accountStatus=active)(accountStatus=shared)(accountStatus=forward)))
result_attribute = uid
result_format = %u

ldap-domains.cf:

version = 3
server_host = localhost
server_port = 389

bind_dn = (redacted)
bind_pw = (redacted)
search_base = ou=DNS,ou=System,dc=(redacted)
query_filter = (associatedDomain=%s)
result_attribute = associatedDomain

ldap-virtual-maps.cf:

version = 3
server_host = localhost
server_port = 389

bind_dn = (redacted)
bind_pw = (redacted)
search_base = ou=Users,dc=(redacted)
query_filter = (&(|(mail=%s)(mailAlternateAddress=%s))(|(accountStatus=active)(accountStatus=shared)))
result_attribute = mailMessageStore

I can’t seem to find the issue. Any hints are highly appreciated. Also, if any further information is needed, I am happy to add it.