I have also recently added an Ads.txt as I got a notification from Google about it. I did see a drop in revenue as well immediately after adding it to the root of my sites.
I added the ads.txt as I got a notification from Google about it, for me this was a few month ago. I saw revenues climb. This month I saw revenues decline. There is no relation between these two events.
The purpose of ads.txt is to prevent scammers from selling ad space on your website. To be specific, a scammer would enter the ad marketplace offering advertisers to show ads on your site, possibly at a discount. Advertiser would agree, ads would be shown on a site that was not yours but was configured to appear as being yours. The scammer would pocket all the revenue and the advertisers ad would never been seen by a single human. Ads.txt prevents this by allowing the ad buyers to query your server to see whether the seller is authorized to sell ad space on your site.
Assuming that your are being scammed by fraudulent ad-sellers selling your inventory, then adding ads.txt should prevent this from occurring and would likely result in an increase in ad revenue. As the advertisers wishing to shows ads on your site could only do so by going through your authorized ad seller (AdSense), your revenue would increase.
Assuming that your are not being scammed. Then it changes nothing.
The only way that a loss in revenue is possible is if you make a mistake in the format or pub numbers of the ads.txt. It is essential that your ads.txt can be accessed by bots (not all bots are bad), and that the information displayed is correct. Google provide tools for testing this and your can always type yourdomain.com/ads.txt to see it.
If the only ad-network on your site is AdSense then all you need is a single line that authorizes AdSense to sell your ad space. You do not need a line for each third party network.
