This confusing behaviour is typical. The underlying reason is that it is up to the carrier to decide what to do with the APN setting - unlike most configurations of this form, you're not actually configuring any connection setting. Instead, you're just passing to the carrier a text string.

The carrier may do a number of things with the APN you've specified:

• Ignore it completely and connect you to the default network.
• Ignore it if it isn't one that it recognises (eg. your case of using a different provider's APN).
• Use it to connect you to the default network, but with an IP from a restricted range.
• Use it to connect you to a private network.

Additionally, the carrier may change the default/override settings at any time, which is why an APN setting might work for a while and then stop working.

There's some awfully detailed info here on the confusing way APN settings behave.

This confusing behaviour is typical. The underlying reason is that it is up to the carrier to decide what to do with the APN setting — unlike most configurations of this form, you're not actually configuring any connection setting. Instead, you're just passing to the carrier a text string.

The carrier may do a number of things with the APN you've specified:

• Ignore it completely and connect you to the default network.
• Ignore it if it isn't one that it recognises (e.g. your case of using a different provider's APN).
• Use it to connect you to the default network, but with an IP from a restricted range.
• Use it to connect you to a private network.

Additionally, the carrier may change the default/override settings at any time, which is why an APN setting might work for a while and then stop working.

There's some awfully detailed info here (LTE Quick Reference on ShareTechnote) on the confusing way APN settings behave.