Some points
The air we fly through normally is not always clean and often contains ppm of volcanic residue. Thi sis known by the large high altitude world wide operators who have to maintain planes and restore ash errosion and contamination (windows, leading edges, engine bleed and oil systems etc.
Through the collective data of the OEMs all operator warnings are in place relative to what is very bad and can lead to completely disabling an aircraft and the onset of the symptoms that forewarn this (see your FCOMs). In bertween this and quite, common in major erruptions, Mt Redobt,, St Helens, Pinatuba etc. are events where the engine or aircraft has to be taken out of service at high cost in order to return it to an airworthiness standard.
To both the aviation safety professionals as well as the operators this can be described as a Red (do not fly zone), Yellow (you may have imapct on your operation) and Green (normal wear and tear).
It was always the objective that the volcanolgists and meteorolgists would model the Red zone and Yellow zones make up in PPM and content and advise the operators where they would be at any given time so that safe and effective fleet management could take place. Obviously this did not happen in an organized way. We all share the responsibility for this in not anticpating this and for me this is especially troubling since I bear a large respionsibility in this.
Now we have the operators themselves absorbing the task by flying test flights to at the same time accepting some risk of increased maintenance costs should they encounter Yellow zones for any length of time. Always the intent is to err on the side of safety so do expect some diversions, and Air-turn-backs. Eventually the forecasters will better be able to antcipate these zones and the operations will be able to operate on schedule.
In is in the interests of all aviation interests to devise means to best meet the challenges including supporting both long terms research as well as short term management of risks.