no more 802.11n connection since 2.6.0

### Basic Infos

- [x] This issue complies with the [issue POLICY doc](https://github.com/esp8266/Arduino/blob/master/POLICY.md).
- [x] I have read the documentation at [readthedocs](https://arduino-esp8266.readthedocs.io/en/latest) and the issue is not addressed there.
- [x] I have tested that the issue is present in current master branch (aka latest git).
- [x] I have searched the issue tracker for a similar issue.
- [x] If there is a stack dump, I have decoded it.
- [x] I have filled out all fields below.

#### Platform

- Hardware: ESP-12
- Core Version: 2.51 , 2.6.0, 2.7.4
- Development Env: Arduino IDE 1.8.13
- Operating System: Windows

### Settings in IDE

- Module: Wemos D1 mini r2

### Problem Description

sketch is "Udp NTP Client" included with release core.

I recently noticed that I had 802.11n and 802.11g devices on my network.
and I noticed that those which were connected in 802.11n are old modules on which I did not make any change in their codes and which were compiled with a core version lower than 2.5.0 (2.4.2 for the majorities)

I therefore put the supplied program "Udp NTP Client" and I tested with different versions of the cores, starting from 2.4.2.

up to version 2.5.1 included, it appears in 802.11n on the wifi router of my box.
except since version 2.6.0 until the last 2.7.4, it appears in 802.11g ...

do you have any idea why?