Had the same issue after the update.
The problem is, that the weatherforecast first checks the paid version of the API to get full day forecasts (https://api.openweathermap.org/data/2.5/forecast/daily?id=xxx&cnt=7&units=metric&lang=de&APPID=xxx)

This results in a 401 for me because I’m using a free API token.

Next the weatherforecast module falls back using the 5 day / 3 hour Forecast API by using the same cnt value (https://api.openweathermap.org/data/2.5/forecast?id=xxx&cnt=7&units=metric&lang=de&APPID=xxx). Here the value 7 for cnt returns 7 entries, that’s why there is only a two day forecast.

I’ll try to write a fix.
In the meantime as a quick fix you can change line 297 of weatherforecast.js from:

params += "&cnt=" + (this.config.maxNumberOfDays < 1 || this.config.maxNumberOfDays > 17 ? 7 : this.config.maxNumberOfDays);

to

params += "&cnt=" + (this.config.maxNumberOfDays < 1 || this.config.maxNumberOfDays > 5 ? 40 : this.config.maxNumberOfDays * 8);

Remeber the fallback API provides only a 5 day forecast, that’s why maxNumberOfDays is limited to 5.