How did "shortly" acquire its seemingly illogical meaning?

"Short" and "brief" are synonyms; an event that is "short" or "brief" occurs for a short period of time. Yet "shortly" and "briefly" are not synonyms; an event that shall occur "shortly" shall occur in a short time, while an event that shall occur briefly shall occur for a brief period of time.

The meaning of "briefly" follows the normal rule for forming adverbs from adjectives, but the meaning of "shortly" does not. That is:

• If I stare lecherously, that means my staring is lecherous
• If I devour greedily, that means my devouring is greedy
• If a building collapses spectacularly, that means its collapsing is spectacular
• If I will visit briefly, that means my visiting will be brief, BUT...
• If I will visit shortly, that does NOT mean my visiting will be short, which breaks the pattern

How, and when, did this puzzling state of affairs come to be? It's not even as though English lacks other words to express the meaning "shortly" has - we have the perfectly good words "soon" and "imminently", after all - so how and why did people ever even begin to use "shortly" in this seemingly illogical manner?

One should look at the origins and changes in meanings of both 'short' and 'shortly'.

From the Online Etymological Dictionary:

short (adj.):

Middle English short, from Old English sceort, scort, "of little length; not tall; of brief duration," ... probably from Proto-Germanic **skurta*- (source also of Old Norse skorta "to be short of," skort "shortness;" Old High German scurz "short"), from PIE root *sker- (1) "to cut," on the notion of "something cut off."

And the old senses have persisted (and others, often zero-derivation nouns, added, as with the electrical fault and the stiff drink).

..............................

Again from Etymon:

shortly(adv.)

Middle English shortli, "for a brief time," from Old English scortlice "briefly," also, in late Old English, "in short time; concisely;" see short (adj.) + -ly (2). By 1815 as "curtly, abruptly."

So the original meaning of 'shortly' seems to be 'for a brief duration' rather than 'not too long in the future'.

This does not tell us, though, where the now default meaning, 'not too long in the future' (note that Collins for instance licenses the 'for a brief duration' as still being an acceptable secondary sense) gained the ascendency.

However, Wiktionary tells us that the 'soon' sense was always available, even in Old English:

From Middle English schortly, schortliche, from Old English sċortlīċe (“shortly; before long; soon”), equivalent to short +‎ -ly

(It also adds a third sense, 'snappily'. 'Concisely' is a smallish broadening from 'not speaking at length'; 'speaking briefly'.)

The default meaning is almost certainly 'soon' nowadays, though context can help disambiguate in many cases. It is not unusual for synonyms to develop different though related default meanings where this is convenient. 'Shortly' has gone one way while 'briefly' has stayed with the earlier sense as default.