Not new at all.
I have had them in my Emperor Valerians army for the invasion of Persia in 260. I built the army in 2012.
For primary evidence they Notitia Dignitatum and there are several funeral monuments that list them
A few interesting arguments continue about the actual role of the lanciarii. Brian Campbell believes that lanciarii were associated with the 'sacer comitatus' ("sacred retinue") and was part of the small mobile field force accompanying the emperors, he also argues that the lanciarius ranked above a legionary, but below a guardsmen.
Phil barker suspects that the are light armed troops that performed the skirmishing of a Legio from the first century and later in the period were formed into their own troop type.
Speidel argues that there were two types of legionary in a legion and uses Arrian description of two types of longchopohoroi - fully half of the regular legionaries, who throw their weapons overhead of the other half. Arrian's legions in Cappadocia (XII Fulminata and XV Apollinaris) deployed in 10 rank deep formation, ranks five to eight were armed with lancea, rank nine was archers and ten was horse archers to counter the Alan Cataphracts.
Duncan Head suggests imperial Roman lanciarii had their origins in the republican
Antesignani. Duncan also notes that a similar split among the legionaries can be found in a papyrus detailing donative records of Legio II Traiana in Egypt ca. 300 AD, from which it appears something like 45% of the men mentioned are lanciarii.
I've found a mentioning lancerarii in the battle of Nisibis, 217th AD (Herodian, IV, 10-15), acting only those skirmishers.
Adam Anders PHD Roman Light Infantry and combat, is not convinced
https://orca.cardiff.ac.uk/id/eprint/29039/1/2012andersaophd.pdfFinally there is a detailed chapter on the lanciarii of Legio II Parthica in Ross Cowan's PhD thesis. It is freely available here
https://theses.gla.ac.uk/779/Military Illustrated Magazine 48 (1992) is also available online if you search for it.
I built two units in my army legio II Parthica and Lanciarii Iuniores
https://stormandconquest.blogspot.com/2012/10/lanciarii-iuniores.html?m=0
Cheers
Matt