Not a bug. A feature.
At later stages you have to choose which of your troops to maximize and upgrade right from the start – to accumulate early game RPS, and which you leave behind till you can purchase special upgrade.
The same applies to sequential buffs – while they’re not quite powerful for those units, for AA jeeps, whose special upgrade works as a multiplier – you really should try to max seq. boost from jeeps before any upgrades to AA jeeps.
The jeeps a there, so you can go without loosing important stuff(i.e, when your passibe bonus hight enough, you can pass all upgrades for soldiers,getting 125s. 25rocketman, 5 mortars, 1 jeep – then upgrade jeeps all the way and freely buy AA jeeps)