there seems to be a bug with the winbox.
I see this bug with version 6.37.1 and version 6.38rc24
When I set the wireless band to 2GHz-G/N (to avoid using 802.11b rates) immediatelly the HT-MCS tab is hidden in winbox.
I attached two screenshots to exhibit this issue.
Here everything is ok:
Here the HT-MCS tab is gone with G/N-rates
HT-MCS tab hides when setting Band to 2GHz-G/N
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Re: HT-MCS tab hides when setting Band to 2GHz-G/N
Report to support directly.