That is unusual. Efficiency normally decays as budget rises and most accounts accept it as physics.
The real problem sat underneath: conversion tracking was counting actions that carried no value, and the bidding was trained on them. The algorithm was finding more of the wrong customer, efficiently.
Three things had to be true before any budget moved. The measurement was rebuilt first, the audience narrowed to travellers arriving in the UAE, and the client's team changed the landing page after our review.
A conversion improvement applies to every visitor from every channel, forever, including the ones you have not bought yet. A budget increase applies only to the visitors it buys, and it stops the day you stop paying.
Which is why order matters. If your page converts at one percent and you double the budget, you have bought twice as many people who leave. The only number that moved was the cost.