You didn't state whether the Tamiya gloss black is acrylic or enamel. Makes a big difference.
How long did you let the paint cure before overcoating? Tamiya gloss colors take longer to cure than flats, up to twice as long under some conditions. That would put your minimum "safe" cure time at about forty-eight hours.
You said you wiped down with alcohol before applying Future. What alcohol at what concentration? Tamiya acrylic paints are highly soluble in methyl, ethyl, and 90% isopropyl.
Fish-eye and beading are caused by surface contamination. Both acrylic and enamel gloss colors in the Tamiya line often contain a silicone additive as a gloss enhancer. That's why sanding the surface removed the problem.
If the black is enamel, scrub the surface with methyl, ethyl, or 90% isopropyl on a soft cloth, or you can use a surfactant cleaner like 409 or Simple Green (the latter needs to be rinsed off). I the black is acrylic, sanding is your best option.
Note also that applying Future in multiple thin coats may help. Also, when applied in a heavy coat, Future often fish-eyes horribly—half an hour later it's glass smooth.