[x-bird]

normal, and nothing to worry about at all.
Once you have solid fluid in the filler line from bleeder up into the syringe, it can only force air out from around the threads since the threads are before the opening in the other end of the bleeder. Any air in those thread gaps is going to come outwards and backwards, and not get drawn in.
When you remove the filler line with the reservoir open, fluid comes out the bleeder from gravity, again it's keeping air out.
It's easier for air to move around those threads while using a mity vac, so that sort of shows why that deal is not worth the effort. that leakage is good when pushing, bad when pulling since you never know if it's thread leakage air or air from in the brake system. I pumped air from my system with a mity vac over and over for days and days thinking i must be going nuts or had a cracked hose, fitting, master, banjo bolts, washers ... wasted several weeks on brakes thinking there was a problem. it was just the wrong tool and approach.

Got a picture of brake linkage and master setup?