Birmingham will feel they did more than enough to win the match but could have come away with nothing with the away side much the better following their equaliser.
Japan international Kyogo Furuhashi was given a first start since September and missed a fine chance to put Blues in front when he lifted over after being played in on goal.
Paik Seung-ho also hit the top of the crossbar from a wide free-kick as the home side did much of the running in a keenly fought first 45 minutes.
The hosts had their goal within three minutes of the restart when Neumann’s looping header beat Bazunu moments after the same player had seen a goalbound effort well-blocked by Taylor Harwood-Bellis as the visitors struggled to deal with a deep free-kick.
Thereafter it looked likely Birmingham would add to their goal before the game’s key incident changed things completely.
A ball was played up to the Saints forwards and Alfons Sampsted looked to head it down for Iwata, only for the midfielder to be accidentally blocked from getting to it by Donohue.
Willum Willumsson then failed to control the ball and Southampton moved it in behind the inside left channel for Elias Jelert to cross in for fellow sub Archer to score.
Iwata and Davies were booked for their subsequent protests and Davies was then sent off for throwing the ball away.
Saints pushed on in search of a winner but could not fashion a clear-cut opening despite enjoying the lion’s share of the closing stages.
