I have not seen this latest Osprey publication. But the cover does cause some head scratching.
I concur with the esteemed and wise GurKhan, who also seems to be in a pickle about this.
The disappearance of the spara shield seems to go along with the disappearance of the Persian cavalry using bows as their primary weapon.
The Hoplite shield is shown on sources, used by Duncan in his fine Montvert Achaemenid armies book, illustrated in great detail by Richard Scollins (still the best source), as well as the Osprey book on Persians written by Nick Sekunda, and illustrated in a classic relief way by Simon Chew.
Both authors tend to agree that the sparabara was a thing of the past, along with the "violin" shaped shield.
Persian infantry being run down on the Kinch Tomb reveal a light round peltast shield.
Now all of this pertains to sources we have access to. Who is to know what the Red Sea men were equipped with, or the nameless Asiatic infantry rabble that is mentioned as part of the Persian cavalry forces at the Granicus. These Satrapal militias may have carried older gear, maybe even spara. However the counter argument to "why not?" is also "why bother" because these troops, held in reserve simply were too fragile and worthless to stand in combat, and appear to only be included as "filler" to pad the myriads.
The Uxians (ex Elamites), Babylonians, (Red Sea) Arabs, and Sittacenians (ex Chaldeans), would all seem to formerly be decent candidates for sparabara. In effect they are pre-scripted to just run away before any of their arms or protection came into play, so as to not call attention to whether there were 250k of them (or not).
But if I was a betting man- I would put my money on small or hoplite sized round shields, as well as crescent shields.
Osprey does show Spara:
link
Kinch tomb-maybe Successors but looks like a prodromos skewering a Persian to me.
This from the Sidon Sarcophagus is the main source that at least some front line Persian infantry used hoplite shields:
link
As you can guess- it is difficult to find a place or battle where any Persian infantry are described contesting Macedonian infantry. The only real mention is at the last stand at the Persian Gates- where Persian infantry did defeat the Macedonians, albeit briefly. All other situations are lost in the speculative reconstruction dialogue. Did any Kardakes stand up to the Hypaspists? Did the Royal Guard infantry stand in front of the phalanx? We have a mosaic and a sarcophagus.