Hello everybody!
My first post is here just to fix this for others that might have this problem.
So the things you need to check are as follows:
- You have proper rewrite rules. In my case, I was on NGINX and wanted a sub-folder setup so I had to use the ones in the docs here: Bedrock: Server Configuration | Roots Documentation
you can find there all the other servers and versions there as well.
- Check that you have configured the correct
DOMAIN_CURRENT_SITEconstant in your application.php (or via env vars as you want). I used the settings offered here: Multisite setup?
I don’t really know why these are not in the official docs 
→ they should be
- And the most important, the one that caused most issues for me, is the fact that on multisite, there is no more /wp/ in the wp-admin url, because the rewrite rules handles these.
And also, most importantly, in a multisite scenario, WordPress will no longer consider the wp-config.php constants for WP_SITEURL and WP_HOME. So these will not overwrite the ones in the DB. This means that WordPress will use the WP_SITEURL and WP_HOME that he finds in the wp_options table. So on a clean WP Bedrock multisite install, these settings in wp_options will both be configured as domain.com/wp which will make your default site to have this /wp ending.
So to fix this, if you already followed points 1 and 2, and you’re still seeing the /wp ending for your default site, then go in your mysql and manually edit these options.
I used manual queries from command line like so:
update wp_options set option_value="https://domain.com" where option_id=1;
update wp_options set option_value="https://domain.com" where option_id=2;
In this case option_id=1 is siteurl and option_id=2 is home. This should be the case in all setups. Or you might use option_name=siteurl and option_name=home instead of option_id=1 and 2 respectively.