When you hit a curtain number of suppliers the auto lookup feature changes - apparently for speed?!
Issue - When you have X number of suppliers recorded and active (over 2.5 pages) the look up feature changes apparently to make things easier and quicker - although I contest this and say it makes it harder and more difficult!
First stage lookup allows you to search for any part of a supplier name, second stage after you hit X number of suppliers allows you to look up only the starting part of the name - e.g.;
If you record Sainsbury's supermarket by its official name it is "J Sainsbury's", First stage lookup allows you to type SAIN or BURY and it appears in the drop down list, Second stage lookup changes it that you must search for J SA...
This change does not make things easier, its confusing as there is no evidence to this in the support documents that can at least be easily found (if at all), and even support were perplexed with this issue (support ticket #297575).
Solution- Change the lookup feature to allow any part of the name to be searched for a supplier, just like all other searches in the system when creating purchases