Greetings.
I might have found the root of the cause of connectivity issues that me and other KPN users in The Netherlands are experiencing.
The default APN is set to use both TCP/IPv4 and v6 to connect to the internet. On top of that, it is set to use the following connection types (bearers) transfer data over:

LTE, HSPAP, HSPA, HSUPA, HSDPA, UMTS, EDGE, GPRS, eHRPD, EVDO_B, EVDO_A, EVDO_0, 1xRTT, IS95B, IS95A and NR.
We are in The Netherlands. We only use NR, LTE, and HSPAP. turn the rest off, unless you are in the wilderness and connect to EDGE.
To achieve maximum stability, use an access point with the following settings applied:

Name: KPN/Hi 4G Mobiel Internet
APN: kpn4g.nl
Proxy: none
Port: none
Username: none
Password: none
Server: none
MMSC: none (warning, MMS will not work)
Multimedia message proxy: none
Multimedia message port: none
MCC: 204
MNC: 08
Authentication type: not set
APN Protocol: IPv4
APN roaming protocol: IPv4
Bearer: LTE, NR - switch off the rest
Mobile virtual network operator type and values: none

Important: If you decide or need to use IPv6, then switch the APN protocol to IPv4/6 - AND turn HSPAP on in the bearer's section. Apparently, KPN transmits TCP/IPv6 traffic over HSPAP, which is a technology that is not used anymore in The Netherlands. Hence the stability issues.
You can also change the APN if you experience problems, to:

Name: KPN Internet APN: Internet

The rest adjust accordingly to your needs according to the setting listed above.

Hypothesis: Is TCP/IPv6 traffic being directed through HSPAP, which might cause packet loss? This is a question directly to KPN as I do not have access or rights to access such information.